Reverberator and method for reverberating an audio signal

ABSTRACT

A reverberator for reverberating an audio signal includes a feedback delay loop processor for delaying at least two frequency subband signals representing the audio signal by different loop delays to obtain reverberated frequency subband signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 13/452,351 filed Apr. 20, 2012, which is a continuation ofInternational Application No. PCT/EP2010/064909, filed Oct. 6, 2010,which additionally claims priority from U.S. Application No. 61/253,655,filed Oct. 21, 2009, each of which is incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to a reverberator and amethod for reverberating an audio signal. Further embodiments of thepresent invention relate to an efficient frequency transform domainreverberator with control for arbitrary reverberation times.

Reverberators are used in creating room effect to audio signals. Thereare numerous audio signal processing applications where there is a needto add room effect to the signal, namely early reflections andreverberation. Of these two, the early reflections appear for only avery short time period after the signal itself, and can thus be modelledmore easily, while the reverberation spans over a long time interval andis often audible up to several seconds after the offset of the drysource sound. The long time span brings the design of the reverberatorinto the main focus in systems which necessitate a room effect whilenecessitating low to medium computational cost.

The design goal of the reverberator may be to maximize the perceptualsimilarity to a certain real or virtual space, or to createreverberation that maximizes some other perceptual property to maximizethe listener preference. There exist several algorithms forreverberation, especially for time domain signals, and the design goalalmost is to find a balance where the desired quality is maximallyreached while the computational load is minimized.

Historically, the reverb design has almost entirely focused on timedomain signals. However, in modern audio processing schemes it is verycommon to have the processing in a short time frequency transformdomain, such as in the QMF (quadrature mirror filterbank) domain used inMPEG surround and related technologies, MDCT (modified discrete cosinetransform) domain, used in perceptual audio codecs and STFT (short timeFourier transform) domain which is used in a very wide range ofapplications. While the methods have differences, the common factor isthat the time domain signal is divided into time-frequency tiles, suchas illustrated in FIG. 16. The transform and the inverse transformoperation is typically lossless, and the information about the soundcontent is thus fully contained in both representations. Thetime-frequency representation is used especially in perceptualprocessing of audio since it has closer resemblance to the way humanhearing system processes the sound.

According to the state-of-the-art, there are several existing solutionsin creating reverberation. In “Frequency Domain Artificial Reverberationusing Spectral Magnitude Decay”, Vickers et al, 2006, 121th AESconvention October 2006 and in US 2008/0085008 A1, a known reverbalgorithm functioning in frequency domain is described. Also,“Improvements of Artificial Reverberation by Use of Subband FeedbackDelay Networks”, Igor Nicolic, 112nd AES convention, 2002, proposescreating reverberation in frequency bands.

An infinitely repeating while decaying impulse response of a reverb canbe found in “Artificial Reverberation Based on a Pseudo-Random ImpulseResponse” parts I and II, Rubak & Johansen, 104th AES convention 1998and 106th AES convention 1999 and “Reverberation Modeling Using VelvetNoise”, Karjalainen & Järveläinen, 30th AES conference March 2007.However, the just-mentioned references are about time-domain reverbalgorithms.

In “The Switch Convolution Reverberator”, Lee et al, 127^(th) AESConvention October 2009, an artificial reverberator having low memoryand small computation costs, appropriate for mobile devices, ispresented. The reverberator consists of an equalized comb filter drivinga convolution with a short noise sequence. The reverberator equalizationand decay rate are controlled by low-order IIR filters, and the echodensity is that of the noise sequence, wherein the noise sequence isregularly updated or “switched”. Moreover, several structures forupdating the noise sequence, including a leaky integrator sensitive to asignal crest factor, and a multi-band architecture, are described.

An underlying problem of the existing solutions is that the current mostadvanced efficient reverberation algorithms function in the time domain.However, many applications, which work in the frequency domain,necessitate a reverberation unit. Thus, in order to apply these timedomain algorithms to a signal, the application will have to firstinverse transform the signal before applying the reverberation algorithmin the time domain. This, however, may be impractical depending on theapplication.

Another disadvantage of known time domain reverberators is that they canbe inflexible in terms of designing the reverb to fit a certain set offrequency dependent reverberation times, which is especially importantfor human spatial perception.

SUMMARY

According to an embodiment, a reverberator for reverberating an audiosignal may have: a feedback delay loop processor for delaying at leasttwo different frequency subband signals representing the audio signal bydifferent loop delays to obtain reverberated frequency subband signals,characterized in that the feedback delay loop processor includes foreach frequency subband signal of the at least two frequency subbandsignals a delay line having a plurality of delay line taps providingsignals delayed by different tap delays, a feedback loop connected tothe delay line and a combiner for combining signals output by theplurality of delay line taps.

According to another embodiment, a method for reverberating an audiosignal may have the steps of: delaying at least two different frequencysubband signals representing the audio signal by different loop delaysby using a feedback delay loop processor to obtain reverberatedfrequency subband signals, characterized in that the feedback delay loopprocessor includes for each frequency subband signal of the at least twofrequency subband signals a delay line having a plurality of delay linetaps providing signals delayed by different tap delays, a feedback loopconnected to the delay line and a combiner for combining signals outputby the plurality of delay line taps.

Another embodiment may have a computer program having a program code forperforming the inventive method when the computer program is executed ona computer.

According to an embodiment of the present invention, a reverberator forreverberating an audio signal comprises a feedback delay loop processor.The feedback delay loop processor is configured for delaying at leasttwo different frequency subband signals representing the audio signal bydifferent loop delays to obtain reverberated frequency subband signals.

In embodiments, the frequency-domain signal representation can be in areal or complex domain. Therefore, all operations performed within thereverberator (e.g. delay, sum or multiplication) can be real or complexoperations.

The basic idea underlying the present invention is that theabove-mentioned improved quality/efficient implementation can beachieved when at least two different frequency subband signalsrepresenting the audio signal are delayed by different loop delays. Bysuch a measure, a perceived repetitiveness of the feedback processingcan be avoided or at least reduced, thereby allowing to better maintainthe perceived quality.

According to a further embodiment of the present invention, the feedbackdelay loop processor comprises, for each frequency subband signal, afilter having a filter impulse response, wherein the filter impulseresponse comprises a first block of filter impulse response samples anda second block of filter impulse response samples. Here, the secondblock may be similar to the first block with regard to impulse responsesample spacing. In addition, the first impulse response sample of thesecond block may be delayed from the first impulse response sample ofthe first block by the loop delay for the frequency subband signal. Inthis way, the first blocks and the second blocks of the filter impulseresponses of the filters for the frequency subband signals will bedelayed by the different loop delays.

According to a further embodiment of the present invention, the feedbackdelay loop processor comprises, for each frequency subband signal, asparse filter having a variable filter tap density. By appropriatelyvarying the filter tap density, the filter impulse response of thesparse filter will approximate a predetermined energy envelope.Therefore, it is possible to control the energy envelopes of the impulseresponses of the sparse filters in a frequency dependent way.

According to a further embodiment of the present invention, the feedbackdelay loop processor is configured to attenuate each frequency subbandsignal of the at least two frequency subband signals by an attenuationfactor. Here, the attenuation factor may depend on a predeterminedreverberation time and the loop delay for the frequency subband signal.This allows to subband-wise adjust a gain of the feedback delay loopprocessing such that an energy decay according to a desiredreverberation time can be achieved.

The present invention provides a reverberation structure with animproved efficiency and thus low cost implementation on low-powerprocessors.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1a shows a block diagram of an embodiment of a reverberator forreverberating an audio signal;

FIG. 1b shows an exemplary design of different loop delays for at leasttwo different frequency subband signals according to an embodiment ofthe present invention;

FIG. 1c shows a block diagram of an embodiment of a single subbandreverberation unit for processing an individual frequency subbandsignal;

FIG. 1d shows a schematic illustration of an impulse response of theembodiment of the single subband reverberation unit in accordance withFIG. 1 c;

FIG. 2a shows a block diagram of a further embodiment of a singlesubband reverberation unit with an attenuator within a feedback loop;

FIG. 2b shows a schematic illustration of an impulse response of theembodiment of the single subband reverberation unit in accordance withFIG. 2 a;

FIG. 3 shows a block diagram of a further embodiment of a single subbandreverberation unit with an exponentially decaying noise filter;

FIG. 4 shows a graph of an exemplary filter response functionrepresenting exponentially decaying noise employed by the embodiment ofthe single subband reverberation unit in accordance with FIG. 3;

FIG. 5 shows a graph of an exemplary impulse response of the embodimentof the single subband reverberation unit in accordance with FIG. 3;

FIG. 6 shows a block diagram of a further embodiment of a single subbandreverberation unit with sparse delay line outputs;

FIG. 7 shows a graph of an exemplary filter response functionrepresenting unity impulses with a decaying density employed by theembodiment of the single subband reverberation unit in accordance withFIG. 6;

FIG. 8 shows a graph of an exemplary impulse response of the embodimentof the single subband reverberation unit in accordance with FIG. 6;

FIG. 9 shows a block diagram of a further embodiment of a single subbandreverberation unit with sparse delay line outputs andmultiplication-free phase operations;

FIG. 10 shows a table of exemplary multiplication-free phase operationsemployed by the embodiment of the single subband reverberation unit inaccordance with FIG. 9;

FIG. 11a shows a block diagram of a phase modification unit according toan embodiment of the present invention;

FIG. 11b shows a block diagram of a phase modification unit according toa further embodiment of the present invention;

FIG. 11c shows a block diagram of a phase modification unit according toa further embodiment of the present invention;

FIG. 11d shows a block diagram of a phase modification unit according toa further embodiment of the present invention;

FIG. 12 shows a block diagram of a further embodiment of a singlesubband reverberation unit with serially connected delay line units,intermediate multipliers, delay line inputs and delay line outputs;

FIG. 13 shows a conceptual structure of an embodiment of a reverberatorfor reverberating an audio signal operative in a frequency domain;

FIG. 14 shows a block diagram of an embodiment of a reverberator forreverberating an audio signal with a spectral converter, severaldifferent single subband reverberation units and an output processor;

FIG. 15 shows a block diagram of a further embodiment of a reverberatorfor reverberating an audio signal with orthogonal channel specificoutput vectors; and

FIG. 16 shows a schematic illustration of a continuous short-timetime-frequency transform representation according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows a block diagram of an embodiment of a reverberator 10 forreverberating an audio signal. As shown in FIG. 1a , the reverberator 10comprises a feedback delay loop processor 20 for delaying at least twodifferent frequency subband signals 17 representing the audio signal 5by different loop delays 23 to obtain reverberated frequency subbandsignals 27. The reverberator 10 may also comprise an output processor 30for processing the reverberated frequency subband signals 27 to obtain areverberated audio signal 41.

Referring to FIG. 1a , the reverberator 10 may further comprise afilterbank 12 such as a QMF (quadrature mirror filterbank) forgenerating the at least two different frequency subband signals 17 fromthe original audio signal 5. Moreover, the feedback delay loop processor20 may comprise a first loop delay unit 22-1 for delaying a firstfrequency subband signal 15-1 of the at least two different frequencysubband signals 17 by a first delay to obtain a first reverberatedfrequency subband signal 25-1 and a second loop delay unit 22-2 fordelaying a second frequency subband signal 15-2 of the at least twodifferent frequency subband signals 17 by a second different delay toobtain a second reverberated frequency subband signal 25-2. The firstand the second reverberated frequency subband signals 25-1, 25-2 mayconstitute the reverberated frequency subband signals 27. In theembodiment of FIG. 1a , the output processor 30 of the reverberator 10may be configured to mix the at least two frequency subband signals 17and the corresponding reverberated frequency subband signals 27 toobtain mixed signals 37 and to combine the mixed signals 37 to finallyobtain the reverberated audio signal 41. As depicted in FIG. 1a , theoutput processor 30 may comprise first and second any processing devices32-1, 32-2 and corresponding adding units 34-1, 34-2. The first anyprocessing device 32-1 may be configured to perform any processing onthe first reverberated frequency subband signal 25-1 to obtain a firstprocessed signal 33-1 and the second any processing device 32-2 may beconfigured to perform any processing on the second reverberatedfrequency subband signal 25-2 to obtain a second processed signal 33-2.Here, the any processing operations performed by the first and secondany processing devices 32-1, 32-2 may, for example, be such thatpredetermined multiplication or gain factors will be applied to thefirst and second reverberated frequency subband signals 25-1, 25-2 ofthe reverberated frequency subband signals 27. The first adding unit34-1 may be configured to add the first frequency subband signal 15-1 ofthe at least two different frequency subband signals 17 or a processedversion thereof and the first processed signal 33-1 of the anyprocessing device 32-1 to obtain a first added signal 35-1 and thesecond adding unit 34-2 may be configured to add the second frequencysubband signal 15-2 of the at least two different frequency subbandsignals 17 or a processed version thereof and the second processedsignal 33-2 of the any processing device 32-2 to obtain a second addedsignal 35-2. Here, the first and second added signals 35-1, 35-2 mayconstitute the at least two mixed signals 37.

As depicted in FIG. 1a , the output processor 30 may further comprise atleast two optional any processing devices 44-1, 44-2 for processing thefirst and second frequency subband signal 15-1, 15-2 of the at least twodifferent frequency subband signals 17. The first optional anyprocessing device 44-1 may be configured to perform any optionalprocessing on the first frequency subband signal 15-1 to obtain a firstoptionally processed signal 45-1 and to supply the first optionallyprocessed signal 45-1 to the corresponding adding unit 34-1, while thesecond optional any processing device 44-2 may be configured to performany optional processing on the second frequency subband signal 15-2 toobtain a second optionally processed signal 45-2 and to supply thesecond optionally processed signal 45-2 to the corresponding adding unit34-2. Therefore, the first and second optional any processing devices44-1, 44-2 can essentially be inserted into the parallel (direct sound)paths between the filterbank 12 and the adding units 34-1, 34-2,respectively, for the first and second frequency subband signal 15-1,15-2 of the at least two different frequency subband signals 17. Forexample in binaural processing, the first and second optional anyprocessing devices 44-1, 44-2 can be configured to apply HRTFs (headrelated transfer functions) to the first and second frequency subbandsignal 15-1, 15-2 of the at least two different frequency subbandsignals 17 to obtain the first and second optionally processed signals45-1, 45-2.

Here, the first adding unit 34-1 may be configured to add the firstprocessed signal 33-1 of the any processing device 32-1 and the firstoptionally processed signal 45-1 of the optional any processing device44-1 to obtain the first added signal 35-1, while the second adding unit34-2 may be configured to add the second processed signal 33-2 of theany processing device 32-2 and the second optionally processed signal45-2 of the optional any processing device 44-2 to obtain the secondadded signal 35-2. Here, the first and second added signals 35-1, 35-2may constitute the at least two mixed signals 37.

It is furthermore shown in FIG. 1a that the output processor 30 may alsocomprise a combiner 38 for combining the mixed signals 37 to obtain thereverberated audio signal 41. The combiner 38 of the output processor 30may comprise at least two further any processing devices 36-1, 36-2 anda putting together unit 39. The first further any processing device 36-1may be configured to further process the first mixed signal 35-1 of theat least two mixed signals 37 to obtain a first further processed signal37-1 and the second further any processing device 36-2 may be configuredto further process the second mixed signal 35-2 of the at least twomixed signals 37 to obtain a second further processed signal 37-2.Similar to the first and second any processing devices 32-1, 32-2, thefirst and second further any processing devices 36-1, 36-2 may performthe further any processing operations by applying predeterminedmultiplication or gain factors to the mixed signals 37. The puttingtogether unit 39 of the combiner 38 within the output processor 30 maybe configured to subsequently put together or combine the first andsecond further processed signals 37-1, 37-2 to obtain the reverberatedaudio signal 41 at the output of the reverberator 10. By a processingsuch as performed with the reverberator 10, a reverberated audio signalrepresenting combined reverberated frequency subband signals having acombined or larger bandwidth will be obtained. Essentially, theembodiment of FIG. 1a represents a reverberator for reverberating theaudio signal within a subband domain such as within a QMF domain.

FIG. 1b shows an exemplary design 50 of different loop delays for the atleast two different frequency subband signals according to an embodimentof the present invention. Referring to FIGS. 1a ; 1 b, the reverberator10 may comprise a feedback delay loop processor 54 which may beconfigured so that the loop delay 56-2 for a second frequency subbandsignal 51-2 of the at least two frequency subband signals 53representing a lower frequency band will be larger than the loop delay56-1 for a first frequency subband signal 51-1 of the at least twofrequency subband signals 53 representing a higher frequency band. Inparticular, the feedback delay loop processor 54 may comprise at leasttwo loop delay units 57, wherein a first loop delay unit may beconfigured to delay the first frequency subband signal 51-1 representingthe higher frequency band by the first loop delay 56-1 to obtain a firstreverberated frequency subband signal 55-1 and a second loop delay unitmay be configured to delay the second frequency subband signal 51-2representing the lower frequency band by the second larger loop delay56-2 to obtain a second reverberated frequency subband signal 55-2. Thefirst and second reverberated frequency subband signals 55-1, 55-2 mayconstitute the reverberated frequency subband signals 57. Here, thefeedback delay loop processor 54, the frequency subband signals 53 andthe reverberated frequency subband signals 57 of FIG. 1b may correspondto the feedback delay loop processor 20, the at least two differentfrequency subband signals 17 and the reverberated frequency subbandsignals 27 of FIG. 1a , respectively. In the design of FIG. 1b , thereverberator 10 may comprise an output processor 60, which may beconfigured to process the reverberated frequency subband signals 57 toobtain a reverberated audio signal 61. Here, the output processor 60shown in FIG. 1b may correspond to the output processor 30 shown in FIG.1a , while the reverberated audio signal 61 output by the outputprocessor 60 may correspond to the reverberated audio signal 41 outputby the output processor 30 of FIG. 1a . Therefore, by the design of thedifferent loop delays according to FIG. 1b , the loop delays forsuccessive frequency subband signals of the at least two frequencysubband signals representing increasing frequency bands can be madedecreasing on average such that an improved perceptual quality of areverberation will be obtained.

In embodiments, the loop delays for the successive frequency subbandsignals may, for example, be linearly decreasing or set randomly. Bysetting different loop delays for the at least two different frequencysubband signals, repetition effects of the reverberation can efficientlybe avoided or at least reduced.

FIG. 1c shows a block diagram of an embodiment of a single subbandreverberation unit 100 for processing an individual frequency subbandsignal. The single subband reverberation unit 100 comprises a delay line110, a feedback loop 120 and a combiner 130. As shown in FIG. 1c , thedelay line 110 has a plurality 115 of delay line outputs or delay linetaps representing different delays. The delay line 110 is configured forproviding a delay amount (N). Here, the delay line 110, which is denotedby z^(−N), has a delay line input 105 for the individual frequencysubband signal 101. The feedback loop 120 is connected to the delay line110 and is configured for processing the individual frequency subbandsignal 101 or a delayed version and for feeding the processed signal orthe individual frequency subband signal 101 or a delayed version of theindividual frequency subband signal into the delay line input 105. Thefeedback loop 120 together with the delay line 110 essentiallyrepresents a feedback delay loop introducing a respective delay amount Nto a signal for each roundtrip of the signal circulating within thefeedback loop 120. The combiner 130 is configured for combining signalsoutput by the plurality 115 of delay line outputs or delay line taps toobtain a reverberated frequency subband signal 135. In particular, thecombiner 130 may be used to add the signals output by the plurality 115of delay line outputs together or to first multiply the signals withgain and/or attenuation factors and then add them together or tolinearly combine selected signals output by the plurality 115 of delayline outputs. The single subband reverberation unit 100 of the FIG. 1cembodiment allows to generate a reverberated frequency subband signal135 that has a reverberation corresponding to a reverberation timelarger than the delay amount N.

FIG. 1d shows a schematic illustration of an impulse response 150 of theembodiment of the single subband reverberation unit 100 in accordancewith FIG. 1c . As shown in FIG. 1d , the impulse response 150 comprisesa sequence (P₀, P₁, P₂, P₃, . . . ) of equally spaced pulses separatedby the delay amount N. The equally spaced pulses (P₀, P₁, P₂, P₃, . . .) define a repeating interval 160 corresponding to the delay amount N.Moreover, delayed pulses 155 output by the plurality 115 of delay lineoutputs are distributed within the repeating interval 160 of the equallyspaced pulses (P₀, P₁, P₂, P₃, . . . ). It can be seen in FIG. 1d thatthe equally spaced pulses (P₀, P₁, P₂, P₃, . . . ) of the impulseresponse 150 of the single subband reverberation unit 100 have a sameamplitude, respectively. Referring to FIGS. 1c ; 1 d, the reverberationof the reverberated frequency subband signal 135 may correspond to atime period 165 being larger than the delay amount N.

FIG. 2a shows a block diagram of a further embodiment of a singlesubband reverberation unit 200 with an attenuator 210 within a feedbackloop. The device 200 of FIG. 2a essentially comprises the same blocks asthe apparatus 100 of FIG. 1c . Therefore, identical blocks havingsimilar implementations and/or functions are denoted by the samenumerals. However, the feedback loop 220 of the single subbandreverberation unit 200 in the FIG. 2a embodiment comprises an attenuator210 for attenuating a delayed signal 205. Here, the delayed signal 205is received from the delay line 110 providing a delay amount N for eachfeeding of an attenuated signal 215 or the frequency subband signal 101into the delay line input 105. As shown in FIG. 2a , the attenuator 210is configured to apply an attenuation factor b to the delayed signal205, wherein the attenuation factor b depends on the provided delayamount N and reverberation time T₆₀. As a result of the attenuation bythe attenuator 210 within the feedback loop 220, an impulse response ofthe feedback loop 220 is characterized by a sequence of equally spaceddecaying pulses (P₀, P₁, P₂, P₃, . . . ), wherein the repeating interval160 of the equally spaced decaying pulses (P₀, P₁, P₂, P₃, . . . ) isagain defined by the delay amount N.

FIG. 2b shows a schematic illustration of an impulse response 250 of theembodiment of the single subband reverberation unit 200 in accordancewith FIG. 2a . Referring to the FIG. 2a embodiment, a reverberation ofthe reverberated frequency subband signal 135 may correspond to animpulse response 250 comprising the sequence of equally spaced decayingpulses (P₀, P₁, P₂, P₃, . . . ), wherein delayed pulses 255 output bythe plurality 115 of delay line outputs are distributed within therepeating interval 160 of the equally spaced decaying pulses (P₀, P₁,P₂, P₃, . . . ).

FIG. 3 shows a block diagram of a further embodiment of a single subbandreverberation unit 300 with an exponentially decaying noise filter. Thesingle subband reverberation unit 300 of the FIG. 3 embodimentessentially corresponds to the single subband reverberation unit 200 ofthe FIG. 2a embodiment. As depicted in FIG. 3, the delay line 310, whichmay correspond to the delay line 110 of FIGS. 1c, 2a , comprises aplurality of serially connected delay line units (z^(−D) ¹ , z^(−D) ² ,. . . , z^(−D) ^(N) ) for successively delaying the attenuated signal215 or the frequency subband signal 101 fed into the delay line input105, respectively. Here, each delay line unit 312 of the delay line 310has a respective delay line output 314 for a successively delayedsignal. The combiner 330 of the single subband reverberation unit 300,which may correspond to the combiner 130 of the single subbandreverberation unit 100; 200, comprises a plurality 350 of multiplierseach connected to a corresponding delay line output. In particular, theplurality 350 of multipliers is configured for multiplying eachsuccessively delayed signal output by the plurality 115 of delay lineoutputs with a corresponding filter coefficient of a filter responsefunction h(n), n=1, 2, . . . , N, respectively, to obtain multiplieroutput signals 355.

In embodiments, an individual delay line unit (individual elementarydelay slot) can be denoted by z^(−D) ^(i) , wherein D_(i) (i=1, . . . ,N) is a partial delay amount, which is introduced by the individualdelay line unit. In particular, D₁, D₂, . . . , D_(N) can be the same(z^(−D)) such as 1 (z⁻¹) or can have different values. Thisgeneralization also refers to the other Figures though not explicitlymarked. Here, the partial delay amount D_(i) may correspond to a delayby one sample (time slot), so that delayed pulses output by theplurality of delay line outputs will be spaced closely adjacent to eachother. Specifically, the delay line may comprise a number of individualdelay line units that corresponds to the delay amount N provided by thedelay line consisting of the plurality of serially connected delay lineunits (z^(−D) ^(i) ). According to further embodiments, the delay amountN provided by the delay line may also be obtained when the partial delayamount D_(i) is increased corresponding to a delay by more than onesample, while at the same time, the number of individual delay lineunits is reduced. In this case, the delayed pulses output by theplurality of delay line outputs will be spaced further apart from eachother corresponding to a coarser resolution.

As shown in FIG. 3, the combiner 330 may comprise an adder 360 foradding together the multiplier output signals 355 to obtain thereverberated frequency subband signal 135. According to the embodimentshown in FIG. 3, the combiner 330 may be set so that the filter responsefunction h(n) will have a decaying amplitude characteristics, wherein alength N of the filter response function h(n) is equal to the delayamount N. Moreover, in the FIG. 3 embodiment, the feedback loop 120 ofthe single subband reverberation unit 300 is configured for receiving adelayed signal, which may correspond to the delayed signal 205 of FIG.2a , from an, in processing direction, last delay line unit output 315of the delay line 310. Here, the processing direction is indicated bythe pointing direction of the arrows within the feedback loop 120 andthe delay line 310.

FIG. 4 shows a graph of an exemplary filter response function 400representing exponentially decaying noise employed by the embodiment ofthe single subband reverberation unit 300 in accordance with FIG. 3. Inparticular, the combiner 330 of the single subband reverberation unit300 may be configured to provide a filter response function 400 based onh_(DNF)(n)=noise(n)·a^(n), n=1,2, . . . , N, wherein noise(n) is a noisefunction, and wherein the decaying amplitude characteristics of thefilter response function h_(DNF)(n) is based on an exponentiallydecaying envelope an. The noise function noise(n) and the envelope an ofthe exemplary filter response function h_(DNF)(n) 400 are clearlyvisible in FIG. 4. Moreover, the filter response function h_(DNF)(n) 400is exemplarily shown in a range between 0 and N, wherein this rangecorresponds to a length 405 of the filter response function h_(DNF)(n),which may be approximately equal to the delay amount N provided by thedelay line 310 as shown in FIG. 3. Specifically, the combiner 330 of thesingle subband reverberation unit 300 may be set so that the envelopea^(n) depends on an attenuation a per time slot, wherein the attenuationa per time slot is based on a predefined parameter T₆₀ corresponding tothe reverberation time. By such a measure, the filter response functionh_(DNF)(n) may be adjusted so as to represent a correspondingexponentially decaying energy curve.

The single subband reverberation unit 300 shown in FIG. 3 may alsocomprise an attenuator 340, which may correspond to the attenuator 210shown in FIG. 2a , placed within the feedback loop 120. The attenuator340 of the single subband reverberation unit 300 can be used forattenuating the delayed signal received from the last delay line unitoutput 315 by applying an attenuation factor to the delayed signal foreach roundtrip of the signal within the feedback loop 120. Inparticular, the attenuator 340 of the single subband reverberation unit300 is configured to apply an attenuation factor being equal to b=a^(N)to the delayed signal, wherein a is an attenuation per time slot and Nthe delay amount. Here, the attenuation for each roundtrip of thefeedback loop 120 is performed by multiplying the delayed signal fromthe last delay line output 315 with the attenuation factor b=a^(N).

FIG. 5 shows a graph of an exemplary impulse response 500 of theembodiment of the single subband reverberation unit 300 in accordancewith FIG. 3. As shown in FIG. 5, the impulse response 500 of the singlesubband reverberation unit 300 is characterized by exponentiallydecaying noise 510 with an envelope function a^(n), wherein theattenuation a per time slot may be set according to the predefinedparameter T₆₀.

Specifically, the attenuation factor in the feedback loop (i.e. theattenuation factor b to be applied by the attenuator within the feedbackloop) can be calculated from a desired reverberation time in aparticular frequency band with formulab=a^(N),where b is the resulting attenuation factor in the feedback loop and

${a = 10^{\frac{{- 3} \cdot P}{T_{60}f_{s}}}},$where a is the attenuation per time slot, N is the delay line length(i.e. delay amount provided by the delay line) in a particular frequencyband, P is a downsampling factor of a frequency transform, T₆₀ is thereverberation time and f_(s) is the sample rate. This formulaessentially gives an attenuation factor which corresponds to the givenreverberation time T₆₀.

Bandwise exponentially decaying Gaussian noise is generally consideredto be a good approximation of real diffuse reverberation. This isexactly what is created as a reverberation filter by modulating Gaussiannoise by an envelope that attenuates by a factor a per time slot.Therefore, a reverb FIR (finite impulse response) filter can be designedby a functionh[n]=white[n]·a ^(n),or in a complex domain, for example, by a functionh[n]=white[n]·a ^(n) ·e ^(i2π·rand[n]),respectively, where white(n) is a process generating white noise, n isthe time slot index and rand (n) is a process which generates randomvariables from equal distribution from 0 to 1. In particular, the filterresponse function h_(DNF)(n) shown in FIG. 4, which is employed in theFIG. 3 embodiment, can be generated with this process. FIG. 4exemplarily shows the real part of such a reverberation filter togetherwith its modulation envelope.

FIG. 6 shows a block diagram of a further embodiment of a single subbandreverberation unit 600 with sparse delay line outputs. The singlesubband reverberation unit 600 of FIG. 6 essentially comprises the sameblocks as the single subband reverberation unit 200 of FIG. 2a .Therefore, identical blocks having similar implementations and/orfunctions are denoted by the same numerals. However, the delay line 610of the single subband reverberation unit 600, which may correspond tothe delay line 110 of the single subband reverberation unit 200,comprises a plurality of serially connected delay line units (z^(−D))for successively delaying an attenuated signal 215 or the frequencysubband signal 101 fed into the delay line input 105. In the FIG. 6embodiment, the delay line 610 comprises at least three delay lineoutputs 615, which may correspond to the plurality of delay line outputs115 of FIG. 2a , wherein the delay line outputs 615 are configured sothat a delay between a first delay line output 617-1 and a second delayline output 617-2 will be different from a delay between the seconddelay line output 617-2 and a third delay line output 617-3. Thefeedback loop 120 of the single subband reverberation unit 600 isconfigured for receiving a delayed signal from an, in processingdirection, last delay line unit output 613 of the delay line 610.

Moreover, the feedback loop 120 of the single subband reverberation unit600 comprises an attenuator 640 for attenuating a delayed signal,wherein the delayed signal is received from the last delay line output613 of the delay line 610 providing a delay amount N for each feeding ofan attenuated signal 215 or the audio signal 101 into the delay lineinput 105. In particular, the attenuator 640 may be configured to applyan attenuation factor being equal to b=a^(N) to the delayed signal,wherein a is an attenuation per time slot and N the delay amount. Inaddition, the plurality 615 of delay line outputs may especially beconfigured so that a difference between successive delay pairs will onaverage be increasing. Here, the combiner 630, which may correspond tothe combiner 130 of FIG. 1c , is configured to combine the at leastthree delay line outputs 615 to obtain the reverberated frequencysubband signal 135.

In the embodiment of FIG. 6, each individual delay line unit 612 may beconfigured to introduce a partial delay amount D to a successivelydelayed signal. Here, the number of the individual delay line units andthe partial delay amount D which is introduced to a successively delayedsignal may be set as has been described correspondingly before.

According to the FIG. 6 embodiment, delayed pulses output by the atleast three delay line outputs 615 will be non-uniformly distributedhaving a decaying density characteristics within a repeating intervaldefined by the response of the feedback loop 120. The impulse responseof the single subband reverberation unit 600 with sparse delay lineoutputs essentially corresponds to a sparse filter response function.

FIG. 7 shows a graph of an exemplary filter response function 700representing unity impulses 705 with a decaying density employed by theembodiment of the single subband reverberation unit 600 in accordancewith FIG. 6. It can be seen in FIG. 7 that exemplary unity impulses 705are more densely distributed in a region 710 close to an origin 702 of atime/sample axis 701, while the exemplary unity impulses 705 arebecoming more sparsely distributed for larger times/samples 720 up to aborder 703 of the frame, wherein the frame is defined by times/samplesbetween 0 and N, wherein time/sample N corresponds to the delay amount Nprovided by the delay line 610.

For example, the filter response function h_(sF)(n) 700 may be based onh_(sF)(n)=sparse(n), n=1,2, . . . , N, wherein the plurality 615 ofdelay line outputs as shown in FIG. 6 may be configured based on asparse function “sparse(n)”, which sparsely distributes the unityimpulses 705 with a decreasing density for consecutive time slots. Thefilter response function h_(SF)(n) 700 may especially be set so as torepresent an exponentially decaying energy curve 715. Essentially, FIG.7 displays sparse tap locations of an FIR filter. The curve 715 depictsa modeled average energy decay (E_(SF)). Here, the figure does notinclude phase modifications.

FIG. 8 shows a graph of an exemplary impulse response 800 of theembodiment of the single subband reverberation unit 600 in accordancewith FIG. 6. In FIG. 8, signals (i.e. delayed pulses) output by theplurality 615 of delay line outputs of the single subband reverberationunit 600 are clearly visible. For consecutive feeding of the delay lineinput 105, the delayed pulses are sparsely or non-uniformly distributedwithin a first repeating interval 810 between time/sample 0 and N, asecond repeating interval 820 between time/sample N and 2N and a thirdrepeating interval 830 between time/sample 2N and 3N. Here, therepeating intervals 810, 820, 830 may correspond to the repeatinginterval 160 shown in FIGS. 1d ; 2 b. An overall interval 865 of theimpulse response 800 shown in FIG. 8, which may correspond to the timeperiod 165 shown in FIGS. 1d ; 2 b, corresponds to approximately threetimes the delay amount N. In particular, the impulse response 800 of thesingle subband reverberation unit 600 comprises successively delayedsparse pulses having a decaying density for successive time slots withinthe repeating intervals 810, 820, 830, respectively, wherein thedecaying density corresponds to a characteristic distribution of theunity impulses, such as shown in FIG. 7.

It can also be seen in FIG. 8 that amplitudes/levels of the successivelydelayed sparse pulses 815, 825, 835 within the first, second and thirdrepeating interval 810, 820, 830, respectively, are different from eachother and, in particular, are attenuated with respect to each other.Here, the attenuation can be controlled by a respective attenuationfactor b=a^(N) applicable by the attenuator 640 of the single subbandreverberation unit 600. In the FIG. 6 embodiment, the attenuation factorb=a^(N) may, for example, be controlled so that the amplitudes/levels ofthe successively delayed sparse pulses 815, 825, 835 drop significantlyfrom the first to the second to the third repeating interval 810, 820,830, respectively.

Referring to FIGS. 6; 8, the decaying densities and the attenuation forthe amplitudes/levels of the successively delayed sparse pulses 815, 825835 can especially be controlled with the delay line 610 and theattenuator 640, so that the impulse response 800 of the single subbandreverberation unit 600 (FIG. 8) and the impulse response 500 of thesingle subband reverberation unit 300 (FIG. 5) will essentially have thesame energy decay rate. In particular, the single subband reverberationunit 600 can be realized with much less computational effort as comparedto the single subband reverberation unit 300.

This is because although the reverberation algorithm provided by thesingle subband reverberation unit 300 is conceptually relatively simple,it has overhead in terms of the computational costs. Therefore, acomputationally efficient FIR structure such as provided within thesingle subband reverberation unit 600 is advantageous. The FIG. 6embodiment is particularly based on the argument that the human hearingis insensitive to the fine structure of a decaying diffuse reverb, butsensitive to the energy decay rate. For this reason, one may replace thedecaying amplitude a^(n) of the impulse response 400 in FIG. 4 withunity impulses with decaying density such as in the impulse response 700of FIG. 7 to produce the same average overall energy decay.

The visual difference of the overall responses 500; 800 of a singlefrequency band obtained with the single subband reverberation unit 300and the single subband reverberation unit 600, respectively, can beclearly seen in FIGS. 5; 8. In particular, in FIGS. 5; 8, absolutevalues of the response of a reverb algorithm in one frequency bandperformed with the single subband reverberation units 300; 600 areshown, wherein the short and longterm average energy decay is the samein both responses. Here, phase modifications are not included in thefigures. Both responses 500; 800 repeat in intervals of N samples,although the effect is more visible in FIG. 8.

FIG. 9 shows a block diagram of a further embodiment of a single subbandreverberation unit 900 with sparse delay line outputs andmultiplication-free phase operations. The single subband reverberationunit 900 of FIG. 9 essentially comprises the same blocks as the singlesubband reverberation unit 600 of FIG. 6. Therefore, identical blockshaving similar implementations and/or functions are denoted by the samenumerals. However, the combiner 930 of the single subband reverberationunit 900, which may correspond to the combiner 630 of the single subbandreverberation unit 600, comprises a plurality 950 of phase modificationunits indicated by ‘θ’-blocks. Here, each phase modification unit(θ-block) is connected to an individual delay line output (tap) of theplurality 915 of delay line outputs (taps), which may correspond to theat least three delay line outputs 615 of the single subbandreverberation unit 600 as shown in FIG. 6. In the FIG. 9 embodiment, theplurality 950 of phase modification units is especially configured formodifying phases of the delay line tap output signals, wherein the phasemodification for a first delay line tap output 917-1 can be differentfrom a phase modification for a second delay line tap output 917-2. Byapplying different phase modifications for the plurality 915 of delayline tap outputs, an overall phase variation will be introduced into thereverberated frequency subband signal 135 at the output of the combiner930.

Therefore, although having only sparse delay line outputs withoutmultipliers for generating unity impulses with decaying density alreadyproduces a reasonable result, the quality of the reverberation algorithmcan be greatly increased by adding phase variation to the response. Inparticular, the impulse response obtained with the single subbandreverberation unit 900 as the result of the added phase variation willessentially be characterized by a higher quality as compared to theimpulse response obtained with the single subband reverberation unit600. However, applying arbitrary phase modifications would remove or atleast decrease the previously achieved computational benefit obtainedwith the reverberation algorithm provided by the single subbandreverberation unit 600 as compared to that of the single subbandreverberation unit 300. This, however, can efficiently be avoided byrestricting the phase modifications to k·π/2, where k is an integernumber (k=0, 1, 2, 3 . . . ), so that a phase operation performed by aθ-block reduces to simple feeding of the real and imaginary parts of aninput signal to the real and imaginary parts of the output as shown inthe table of FIG. 10.

FIG. 10 shows a table 1000 of exemplary multiplication-free phaseoperations employed by the embodiment of the single subbandreverberation unit 900 in accordance with FIG. 9. In particular, thefirst column 1010 of the table 1000 represents multiplication-free phaseoperations k·π/2 for k=0 (1012), k=1 (1014), k=2 (1016) and k=3 (1018),respectively, each having a periodicity of k·2π. Moreover, the secondand third column 1020, 1030 of the table 1000 represent the real part(‘output real part’) and imaginary part (‘output imaginary part’) of theoutput, which are directly related to the real part (′input real′) andimaginary part (‘input imag’) of the input signal, for the correspondingmultiplication-free phase operations (lines 1012, 1014, 1016, 1018).

FIGS. 11a ; 11 b; 11 c; 11 d show block diagrams of differentembodiments of a phase modification unit 1110; 1120; 1130; 1140, whichmay correspond to a phase modification unit of the plurality 950 ofphase modification units employed by the single subband reverberationunit 900 shown in FIG. 9. In particular, the plurality 950 of phasemodification units may be configured to be operative on the delay linetap output signals, wherein each phase modification unit 1110; 1120;1130; 1140 of the plurality 950 of phase modification units may comprisea first phase modification unit input 1112-1; 1122-1; 1132-1; 1142-1 fora real part of a respective delay line tap output signal or a secondphase modification unit input 1112-2; 1122-2; 1132-2; 1142-2 for animaginary part of the respective delay line tap output signal and afirst phase modification unit output 1114-1; 1124-1; 1134-1; 1144-1 forthe real part of a phase modified output signal or a second phasemodification unit output 1114-2; 1124-2; 1134-2; 1144-2 for theimaginary part of the phase modified output signal.

In FIG. 11a , the first phase modification unit input 1112-1 is directlyconnected to the first phase modification unit output 1114-1 and thesecond phase modification unit input 1112-2 is directly connected to thesecond phase modification unit output 1114-2.

In FIG. 11b , the second phase modification unit input 1122-2 isdirectly connected to the first phase modification unit output 1124-1and the first phase modification input 1122-1 is connected to aninterconnected sign inverter 1125, which is connected to the secondphase modification unit output 1124-2. Therefore, according to the FIG.11b embodiment, the real part of the phase modified output signal willbe based on the imaginary part of the respective delay line tap outputsignal and the imaginary part of the phase modified output signal willbe based on a sign-inverted real part of the respective delay line tapoutput signal.

In FIG. 11c , the first phase modification unit input 1132-1 isconnected to an interconnected sign inverter 1135-1, which is connectedto the first phase modification unit output 1134-1 and the second phasemodification unit input 1132-2 is connected to an interconnected signinverter 1135-2, which is connected to the second phase modificationunit output 1134-2. Therefore, according to the FIG. 11c embodiment, thereal part of the phase modified output signal will be based on asign-inverted real part of the respective delay line tap output signaland the imaginary part of the phase modified output signal will be basedon a sign-inverted imaginary part of the respective delay line tapoutput signal.

In FIG. 11d , the first phase modification unit input 1142-1 is directlyconnected to the second phase modification unit output 1144-2 and thesecond phase modification unit input 1142-2 is connected to aninterconnected sign inverter 1145, which is connected to the first phasemodification unit output 1144-1. Therefore, according to the FIG. 11dembodiment, the imaginary part of the phase modified output signal willbe based on the real part of the respective delay line tap output signaland the real part of the phase modified output signal will be based on asign-inverted imaginary part of the respective delay line tap outputsignal.

The possible phase operations (phase modifications) performed by thedifferent phase modification units 1110; 1120; 1130; 1140 can bereferred to as being multiplication-free, because the output (i.e. thephase modified output signal) can be directly derived from the inputsignal (i.e. the delay line output signal) as has been previouslydescribed without necessitating application of a (complex) phasemultiplier to the signal. The phase modification units 1110; 1120; 1130;1140 therefore represent computationally efficient phase modificationunits.

FIG. 12 shows a block diagram of a further embodiment of a singlesubband reverberation unit 1200 with serially connected delay line units(z^(−D)), intermediate multipliers 1260, delay line (tap) inputs 1209and delay line (tap) outputs 1211. As shown in FIG. 12, the delay line1210 of the single subband reverberation unit 1200 comprises a pluralityof serially connected delay line units (z^(−D)) for successivelydelaying an attenuated signal or the audio signal represented by thefrequency subband signal 1201 fed into different delay line inputs,respectively, wherein each delay line unit of the delay line 1210 has arespective delay line output for a successively delayed signal.Furthermore, the single subband reverberation unit 1200 comprises aplurality 1260 of intermediate multipliers each connected with a delayline output 1207 of a first delay line unit 1205 and a correspondingdelay line input 1213 of a second consecutive delay line unit 1215. Inparticular, the plurality of serially connected delay line units(z^(−D)) of the delay line 1210 shown in FIG. 12 may correspond to theplurality of serially connected delay line units (z^(−D)) of the delayline 610 shown in FIG. 9.

In the FIG. 12 embodiment, the plurality 1260 of intermediatemultipliers is especially adjusted for multiplying successively delayedsignals output from the plurality of serially connected delay line units(z^(−D)) with intermediate attenuation factors to obtain intermediatemultiplier output signals and for supplying the intermediate multiplieroutput signals to the combiner 1230, which may correspond to thecombiner 130 of FIG. 1c , and to corresponding delay line inputs ofconsecutive delay line units within the delay line 1210. Here, theintermediate multipliers 1260 may, for example, be configured as realmultipliers. The feedback loop 1220, which may correspond to thefeedback loop 120 of FIG. 1c , may be configured for receiving a delayedsignal from a last intermediate multiplier output 1265 of the plurality1260 of intermediate multipliers, wherein the delayed signal from thelast intermediate multiplier output 1265 will have an attenuationcorresponding to an effective attenuation factor based on the number ofthe intermediate multipliers 1260 and the individually appliedintermediate attenuation factors. In particular, the plurality 1260 ofintermediate multipliers may be configured to provide an effectiveattenuation factor corresponding to the attenuation factor (b=a^(N))applied by the feedback loop 120 such as within the single subbandreverberation unit 900 shown in FIG. 9. The single subband reverberationunit 1200 may also comprise a plurality 1250 of phase modificationunits, which may correspond to the phase modification units 950 shown inFIG. 9.

Referring to the embodiment of FIG. 12, the partial delay amount Dintroduced by each delay line unit of the plurality of seriallyconnected delay line units (z^(−D)) may correspond to a specific delayby one sample or time slot. In the FIG. 12 embodiment, the plurality ofdelay line output taps corresponding to the delay line outputs 1211 maynot be fully populated. This means that only some output taps of theplurality of serially connected delay units may be connected to thecombiner 1230. In addition, the plurality of intermediate multipliers1260 may also not be fully populated.

According to the FIG. 12 embodiment, at least two delay line units 1215,1218 of the plurality of serially connected delay line units (z^(−D))may have corresponding delay line inputs 1213, 1217 for receiving theaudio signal represented by the frequency subband signal 1201 inparallel. Here, the frequency subband signal 1201 shown in FIG. 12 maycorrespond to the frequency subband signal 101 shown in FIG. 1 c.

In the embodiment of FIG. 12, the audio signal may consist of severalinput audio channels Ch₁, Ch₂, Ch₃ . . . , e.g., denoted by ‘L’ (left),‘R’ (right) and ‘C’ (center). Moreover, each input audio channel of theseveral input audio channels comprises the frequency subband signal 1201of a plurality 1203 of different frequency subband signals.

As depicted in FIG. 12, the several input audio channels Ch₁, Ch₂, Ch₃,. . . (e.g. L, R, C) can be processed differently by preconnected phasemodification units before being fed into corresponding delay line inputsof the plurality of serially connected delay line units (z^(−D)). Here,the plurality 1240 of preconnected phase modification units may beconfigured to apply multiplication-free phase operations which aredifferent for the different input audio channels (Ch₁, Ch₂, Ch₃, . . .).

Therefore, in embodiments, the single subband reverberation unit 1200may be configured for preprocessing a respective frequency subbandsignal of the several input audio channels (L, R, C) differently toobtain different preprocessed signals. In particular, the respectivefrequency subband signal of the several input audio channels (L, C, R)can be preprocessed by using different phase modification units 1240 forapplying different phase modifications for several input audio channelsL, C, R before feeding the preprocessed signals into corresponding delayline inputs of the plurality of serially connected delay line units(z^(−D)).

Specifically, as can be seen in FIG. 12, the single subbandreverberation unit 1200 may further comprise a plurality 1240 ofconnected phase modification units (θ-blocks) each connected to acorresponding delay line input of the plurality of serially connecteddelay line units (z^(−D)), so that phase modifications will be appliedto the frequency subband signal 1201 that can be injected in parallelinto different delay line inputs. Here, it is to be noted that the phasemodification units 1240, 1250 may correspond to efficient phasemodification units such as described in FIG. 11.

According to further embodiments, the preprocessed signals for theseveral channels (L, R, C) of the audio signal may be added prior toinjecting same into the corresponding delay line inputs 1213, 1217. Suchan adding operation is exemplarily indicated in FIG. 12 by ‘+’-symbols1242 being operative on the L, C, R-channels.

According to further embodiments, the delay line 1210 may be configuredso that a sum of a number of delay line inputs 1209 for receiving theaudio signal 1201 and a number of the delay line outputs 1211 will besmaller than a number of individual elementary delay slots of the delayline 1210.

At the output of the combiner 1230 as shown in FIG. 12, a reverberatedfrequency subband signal 1235 of a plurality of different reverberatedfrequency subband signals can be obtained, wherein the reverberatedfrequency subband signal 1235 may correspond to the reverberatedfrequency subband signal 135 of the previous embodiments.

In other words, the audio channels (L, R, C) of the audio signal may bespectrally decomposed into a plurality of different frequency subbandsignals the exemplary single subband reverberation unit 1200 isoperative on. Therefore, FIG. 12 essentially relates to a specificstructure of a frequency-band single subband reverberation unit withdelay line inputs (input taps), delay line outputs (output taps) andintermediate attenuation factors. Here, the phase modification units mayalso be zero multipliers.

In embodiments, a frequency domain reverberation algorithm realized withthe frequency-band single subband reverberation unit can be based onarbitrary injection of the input signals from multiple channels into anypoint of the delay line. It can also be used to generate multiple outputchannels from the pickups of the delay line. According to furtherembodiments, the efficient phase modification units and the realmultipliers within the reverberation structure may be replaced bytime-variant or invariant complex multipliers. Moreover, the order ofthe delay line units, intermediate multipliers (gains), pickup points,and entry points may be interchangeable. In particular, when thechannel-specific injection vectors are configured to be orthogonal, thesingle subband reverberation unit will be enabled to treat coherent andincoherent parts of the input signals equally. In the case that theoutput weighting vectors are configured to be orthogonal, incoherentoutput channels can be produced. Here, the output weighting vectors maycorrespond to attenuated (weighted) signals output by the plurality ofintermediate multipliers each placed after its corresponding delay lineunit. In case the injection vectors are configured to be orthogonal tothe output weighting vectors, energy peaks at the beginning of theimpulse response repetitions may be prevented.

According to further embodiments, the in-one-loop energy decay may becontrolled by adjusting the gains between the delay line units and/or byhaving the density of the output pickups reduced. However, regardless ofthe applied method, the target is to obtain an energy decay rateaccording to a given reverberation time.

In other words, the reverberation structure may utilize the possibilityto inject the input signal with phase modifications into the delay line.Here, it can be beneficial to inject the input signal in parallel intothe delay line, because the impulse response of the system denses by afactor corresponding to the number of the delay line inputs (inputtaps). This especially allows reduction of the delay line outputs(output taps), and permits to have equal impulse response density withless memory excesses and additions. Optimally, the delay length in eachfrequency band may be adjusted to be in the same range as the number ofinput taps times the number of output taps. According to furtherembodiments, the input and the output tap positions can be randomlydistributed using a uniform distribution. In addition, both the overalldelay length and the input and output tap positions can be different ineach frequency band. A further approach is to make use of realmultipliers between the delay line units to provide an energy decayaccording to the reverberation time.

Since the embodiment of FIG. 12 has much computational overhead, it canbe reduced to a number of specific and efficient reverberationstructures. One of these is, for example, the sparse filter structuredescribed in the FIG. 9 embodiment.

The above different embodiments (FIGS. 1c ; 2 a; 3; 6; 9; 12) wererelated to single subband reverberation units being operative on asingle or individual frequency subband signal of the at least twodifferent frequency subband signals, while, in the following, differentembodiments of reverberators being configured to differently process theat least two different frequency subband signals will be described.

FIG. 13 shows a conceptual structure of an embodiment of thereverberator 1300 operative in a frequency domain. The reverberator 1300of FIG. 13 can especially be used to perform the reverberation algorithmin frequency bands. In particular, the reverberator 1300 shown in FIG.13 may correspond to the reverberator 10 shown in FIG. 1a . Here, it isto be noted that the reverberator 1300 of FIG. 13 may comprise aplurality of single subband reverberation units as described previously,wherein each single subband reverberation unit of the plurality ofsingle subband reverberation units may be operative on an individualfrequency subband signal of a plurality of frequency subband signals,and wherein the plurality of single subband reverberation units may beconfigured to process the frequency subband signals differently, toobtain a plurality 1335 of reverberated frequeny subband signals.

Referring to the embodiment of FIG. 13, the reverberator 1300 comprisesa feedback delay loop processor 1320, which may correspond to thefeedback delay loop processor 20 shown in FIG. 1a . Optionally, thereverberator 1300 shown in FIG. 13 may also comprise a first spectralconverter 1310, which may correspond to the filterbank 12 of thereverberator 10 shown in FIG. 1a , and a second spectral converter 1340,which may correspond to the output processor 30 of the reverberator 10shown in FIG. 1a . Here, the first and second spectral converters 1310,1340 are indicated by “time-frequency transform (optional)” and “inversetime-frequency transform (optional)”, respectively. The first spectralconverter 1310 may be configured for converting the audio signal 1301into a spectral representation having a plurality 1315 of differentfrequency subband signals. Here, the audio signal 1301 and the plurality1315 of different frequency subband signals in the embodiment of FIG. 13may correspond to the audio signal 5 and the at least two differentfrequency subband signals 17 in the embodiment of FIG. 1a . As depictedin FIG. 13, the feedback delay loop processor 1320 may comprise, foreach frequency subband signal 1317 of the plurality 1315 of differentfrequency subband signals, a feedback loop 1350 and a diffuseness filter1330 having a plurality of delay line taps. It can be seen in FIG. 13that the feedback loop 1350 comprises a delay element 1352 determiningthe loop delay for the frequency subband signal to obtain a feedbacksignal 1353. In particular, the feedback loop 1350 may comprise an adder1354 for adding the frequency subband signal 1317 and the feedbacksignal 1353. As can be seen in FIG. 13, the adder 1354 is connected tothe diffuseness filter 1330. Specific to the embodiment of FIG. 13 isthat the delay elements of the feedback loops can be different for theat least two different frequency subband signals (signals 1315).

According to further embodiments, the feedback delay loop processor 1320of the reverberator 1300 may comprise a feedback loop 1350 for eachfrequency subband signal 1317 of the at least two frequency subbandsignals, wherein the feedback loop 1350 for a frequency subband signal1317 may comprise a delay element 1352 and additionally, an attenuator1356. Here, the delay elements and also the attenuators may be differentfor the at least two different frequency subband signals.

The second spectral converter 1340 of the reverberator 1300 mayoptionally be used to combine the plurality of reverberated frequencysubband signals 1335 to obtain the reverberated audio signal 1341 havinga combined bandwidth. The reverberated audio signal 1341 obtained withthe reverberator 1300 of FIG. 13 may correspond to the reverberatedaudio signal 41 of the reverberator 10 of FIG. 1 a.

In other words, a single subband reverberation unit of the reverberator1300 or frequency domain reverberation structure may comprise a(decaying) impulse train generator (feedback loop 1350) and a(short-time) diffuseness filter 1330 enclosed within two (optional)spectral converters 1310, 1340 for performing a time-frequency transformand an inverse time-frequency transform, respectively. The transformoperation (blocks 1310, 1340) are optional and shown for illustrationpurposes only since the audio signal in the application may be alreadyin the frequency transform domain. In the transform domain, the order ofthe processing blocks is interchangeable since the processing is linear.All factors included can be different in different frequency bands.Here, the different frequency bands are illustrated by dotted lines atthe output of the time-frequency transform converter 1310 and the inputof the inverse time-frequency transform converter 1340, respectively.

As described previously, the reverberation structure (reverberator 1300)includes a plurality of different one-input-one-output reverberationunits and therefore works for multiple inputs and outputs or differentfrequency subband signals. Conceptually, creating multiple outputs canbe achieved by having multiple diffuseness filters that are mutuallyincoherent. In FIG. 13, the decaying impulse train generator 1350(feedback loop) may be configured to create an infinite exponentiallydecaying sparse equal interval response which defines the repeatinginterval of the reverberation in that particular band. The diffusenessfilter 1330, which may be an FIR (finite impulse response) or IIR(infinite impulse response) filter structure, may in turn be used tocreate a short-time diffuse characteristics to the response. Thisstructure allows the diffuseness filter 1330 to be short and thuscomputationally efficient. The feedback loop makes the overall responseinfinitely attenuating, and the diffuseness filter makes the short timeenvelope attenuating according to the same factor.

There is no specific constraint to the delay line length of thediffuseness filter. In embodiments, the design goal is to make thelength of the delay line as short as possible so as to allow for aminimum memory usage and computational cost in the diffuseness filter,while keeping the negative perceptual effect of the repeating structureminimal.

The diffuseness filter can be designed in many ways. It can, forexample, be designed as an “ideal reverberation” diffuseness filter inthe form of a short-time diffuse filter by decaying white noise (designmethod of FIG. 3) or as an efficient diffuseness filter by means of asparse filter with decaying density and unity gains withmultiplication-free phase operations (design method of FIG. 9). Here,the diffuseness filter design method of FIG. 9 is by informal listeningperceptually equal to the method of FIG. 3, while having significantcomputational savings. Thus, the method of FIG. 9 may be advantageouscompared to the method of FIG. 3. In particular, the sparse filter-basedimplementation such as within the embodiments of FIGS. 6; 9; 12represent a more practical implementation of a reverberation algorithm.

In embodiments, the reverberation structure essentially uses a commondelay line, which is shared by the feedback delay loop and thediffuseness filter (e.g. an FIR sparse filter). Other types ofdiffuseness filters can also be built similarly.

Referring to the embodiments of FIGS. 3; 6; 9; 12, the delay lineincluding the plurality of serially connected delay line units mayconsist of at least 15, advantageously at least 20, and less than 200,advantageously less than 100 individual delay line units (delay lineslots).

According to further embodiments, the diffuseness filter 1330 shown inFIG. 13 or the delay line 110 shown in FIG. 1c are typicallycomplex-valued devices.

FIG. 14 shows a block diagram of an embodiment of a reverberator 1400for reverberating an audio signal with a spectral converter, a feedbackdelay loop processor including several different single subbandreverberation units and an output processor. As shown in FIG. 14, thereverberator 1400 comprises the spectral converter 1410, the feedbackdelay loop processor 1420 and the output processor 1430. Here, thespectral converter 1410, the feedback delay loop processor 1420 and theoutput processor 1430 of the reverberator 1400 shown in FIG. 14 maycorrespond to the filterbank 12, the feedback delay loop processor 20and the output processor 30 of the reverberator 10 shown in FIG. 1a .The spectral converter 1410 may be configured for converting the audiosignal 1401, which may correspond to the audio signal 5 of FIG. 1a ,into a plurality 1415 of different frequency subband signals, which maycorrespond to the at least two different frequency subband signals 17 ofFIG. 1a . In the embodiment of FIG. 14, the feedback delay loopprocessor 1420 comprises a plurality 1421 of single subbandreverberation units, which are configured for processing the differentfrequency subband signals 1415, to obtain reverberated frequency subbandsignals 1425. In particular, a first single subband reverberation unit1422 of the feedback delay loop processor 1420 may be configured toprovide a first total delay amount N₁ for a first frequency subbandsignal 1417-1 of the plurality 1415 of different frequency subbandsignals to obtain a first reverberated frequency subband signal 1427-1,while a second single subband reverberation unit 1424 of the feedbackdelay loop processor 1420 may be configured to provide a seconddifferent total delay amount N₂ for a second frequency subband signal1417-2 of the plurality 1415 of different frequency subband signals toobtain a second reverberated frequency subband signal 1427-2. The outputprocessor 1430 may be configured for processing the reverberatedfrequency subband signals 1425 to obtain a reverberated audio signal1435 such as described previously. Here, the reverberated frequencysubband signals 1425 and the reverberated audio signal 1435 obtained atthe output of the output processor 1430 may correspond to thereverberated frequency subband signals 27 and the reverberated audiosignal 41 at the output of the output processor 30 shown in FIG. 1a ,respectively.

The spectral converter 1410 may, for example, be configured as a QMFanalysis filterbank or for performing a short-time Fourier transform(STFT), while the output processor 1430 may, for example, be configuredas a QMF synthesis filter bank or for performing an inverse short-timeFourier transform (ISTFT).

In embodiments, the frequency-domain signal representation can be in areal or complex domain. Therefore, all operations performed within thereverberator (e.g. delay, sum or multiplication) can be real or complexoperations.

According to further embodiments, the spectral converter 1410 orfilterbank 12 may also be implemented as a real-valued device. Possibleapplications of such a real-valued filterbank can, for example, bemodified discrete cosine transforms (MDCT) in audio coding, or low-powermodes in MPEG Surround, where a lower part of the QMF bands might becomplex, and higher bands might be only real-valued. In such scenarios,there could be environments where at least part of the subbands are onlyreal-valued and where it would be beneficial to apply a reverb likethis. In these cases, the signal is real and the possible phasemodifications (e.g. efficient phase modifications such as described inFIGS. 11a to d ) are only 1 and −1, corresponding to a multiplication ofthe real signal by multipliers 1 or −1, respectively.

By using different total delay amounts (N₁≠N₂) for the plurality 1415 offrequency subband signals, a repetitiveness of the impulse response canbe significantly reduced due to different resulting repeating intervalsfor the different frequency subband signals.

Referring to the embodiments of FIGS. 1c and 14, the feedback delay loopprocessor 1420 may comprise for each frequency subband signal of the atleast two frequency subband signals 1415 a delay line 110 having aplurality 115 of delay line taps providing signals delayed by differenttap delays, a feedback loop 120 connected to the delay line 110 and acombiner 130 for combing signals output by the plurality 115 of delayline taps, to obtain the reverberated frequency subband signals 1425. Inparticular, the delay line 110 is configured to provide a total delayamount which is higher than the highest tap delay. This total delayamount essentially determines the loop delay for the frequency subbandsignal. As depicted in FIG. 14, the total delay amounts N₁, N₂ providedby the first and second signal subband reverberation units 1422, 1424 ofthe feedback delay loop processor 1420 are different for the at leasttwo different frequency subband signals 1415.

According to further embodiments, the feedback delay loop processor 20of the reverberator 10 may comprise, for each frequency subband signal,a filter having a filter impulse response, such as the impulse response800 shown in FIG. 8. As described previously, the filter impulseresponse 800 comprises a first block 815 of filter impulse responsesamples and a second block 825 of filter impulse response samples. Here,the second block 825 is similar to the first block 815 with regard toimpulse response sample spacing, while a first impulse response sample821 of the second block 825 will be delayed from a first impulseresponse sample 811 of the first block 815 by the loop delay for thefrequency subband signal. Moreover, the loop delay for the frequencysubband signal provided by the filter essentially corresponds to a delayamount N defined by the first impulse response sample 821 of the secondblock 820 and the first impulse response sample 811 of the first block815.

Thereby, at the output of the feedback delay loop processor 20, aplurality of different first and second blocks of filter impulseresponse samples for the at least two different frequency subbandsignals will be obtained. Specifically, the first blocks and the secondblocks of the filter impulse responses of the filters for the frequencysubband signals will be delayed by the different loop delays for the atleast two different frequency subband signals.

Referring to the embodiments of FIGS. 6 and 14, the plurality 615 (115)of delay line taps may comprise a first part 619-1 of delay line tapsand a second subsequent part 619-2 of delay line taps. In embodiments,the delay line 610 (110) of a single subband reverberation unit may beconfigured so that an average gap size between the taps of the secondpart 619-2 will be larger than the average gap size between the taps ofthe first part 619-1. Here, the average gap size corresponds to anaverage over successive delays between respective delay line taps of theplurality 615 (115) of delay line taps in the first or second subsequentpart 619-1, 619-2 of delay line taps, respectively.

Referring to the embodiments of FIGS. 1c ; 13 and 14, the diffusenessfilter 1330 of the reverberator 1300 shown in FIG. 13 or the delay line110 connected to the feedback loop 120 and the combiner 130 canespecially be configured as a sparse filter 600 which is exemplarilyshown in FIG. 6. As has been described before, the sparse filter 600 mayhave a filter density that can be varied in such a way that a filterimpulse response (e.g. impulse response 700 of FIG. 7) of the sparsefilter 600 will approximate a predetermined energy envelope (e.g. energyenvelope 715 of FIG. 7).

According to further embodiments, the sparse filter may be implementedas in the embodiment of FIG. 9 (sparse filter 900) comprising aplurality 950 of phase modification units, wherein each phasemodification unit of the plurality 950 of phase modification units isdirectly connected to an individual delay line tap of the plurality 915of delay line taps, and wherein each phase modification unit isconfigured to apply a multiplication-free phase operation to acorresponding signal output by the individual delay line tap. Inembodiments, the multiplication-free phase operation provided by arespective phase modification unit of the plurality 950 of phasemodification units may, for example, be performed according to Table1000 of FIG. 10. Here, the respective phase modification unit may beconfigured as an efficient phase modification unit as shown in FIGS.11a-d . Referring to FIGS. 1c ; 13 and 14, the diffuseness filter 1330or the delay line 110 are typically complex-valued devices forseparately processing real and imaginary parts of a complex signalrepresenting the audio signal. Therefore, by using these complex-valueddevices, the efficient phase modification units 1110; 1120; 1130; 1140as shown in FIGS. 11a-d can be realized.

Referring to the embodiments of FIGS. 1a and 2a , the feedback delayloop processor 20 of the reverberator 10 may be configured to attenuateeach frequency subband signal of the at least two frequency subbandsignals 17 by an attenuation factor b. As described before, theattenuation factor b may depend on a predetermined reverberation timeT₆₀ and the loop delay for the frequency subband signal according toembodiments of the present invention. By such a measure, differentattenuation factors for the at least two different frequency subbandsignals 17 can be applied by the feedback delay loop processor 20.

FIG. 15 shows a block diagram of a further embodiment of a reverberator1500 with orthogonal channel specific output vectors. In the embodimentof FIG. 15, the reverberator 1500 may comprise at least two spectralconverters 1510-1, 1510-2 for a first and a second channel 1501-1,1501-2 (Ch_(in,1), Ch_(in,2)) of a plurality of input audio channels(Ch_(in,1), Ch_(in,2), . . . ), wherein the at least two spectralconverters 1510-1, 1510-2 may be configured as analysis filterbanks(e.g. filterbank 12 of FIG. 1a ) to spectrally decompose the twochannels 1501-1, 1501-2 into a first and a second plurality 1515-1,1515-2 of different frequency subband signals, respectively. As shown inFIG. 15, a feedback delay loop processor 1520 (e.g. feedback delay loopprocessor 20 of FIG. 1a ) may comprise a plurality 1550 of adders thatcan be used for adding corresponding frequency subband signals of thefirst and the second plurality 1515-1, 1515-2 of frequency subbandsignals together, to obtain added signals 1555 and for feeding the addedsignals 1555 into corresponding inputs of a plurality 1521 of singlesubband reverberation units. In particular, a single subbandreverberation unit of the plurality 1521 of single subband reverberationunits may comprise a delay line filter including a delay line 1526providing at least two different filter tap positions 1522, 1524 forfrequency subband signals 1525-1 of a first output audio channelCh_(out,1) and for frequency subband signals 1525-2 of a second outputaudio channel Ch_(out,2), respectively.

Furthermore, the reverberator 1500 may comprise two output processors1530-1, 1530-2 for providing a first and a second output channel 1535-1,1535-2 (Ch_(out,1), Ch_(out,2)) of an output audio signal, wherein thetwo output processors 1530-1, 1530-2 may be configured as synthesisfilterbanks (e.g. QMF synthesis filterbanks). In particular, the firstoutput processor 1530-1 may be set to synthesize a first plurality1525-1 of signals output by a first delay line output or filter tapposition 1522 of the plurality 1521 of single subband reverberationunits, while the second output processor 1530-2 may be set to synthesizea second plurality 1525-2 of signals output by a second different delayline output or filter tap position 1524 of the plurality 1521 of singlesubband reverberation units.

Referring to the FIG. 15 embodiment, the audio signal 5 has theplurality of different input audio channels Ch_(in,1), Ch_(in,2), . . ., wherein each input audio channel has at least two different frequencysubband signals (signals 1515-1, 1515-2). Specifically, the delay line1526 of the delay line filter as part of the feedback delay loopprocessor 1520 may comprise filter tap positions or phase modificationunits connected to at least some of the filter tap positions. Thefeedback delay loop processor 1520 further comprises a first outputconfiguration for frequency subband signals 1525-1 of a first outputaudio channel 1535-1, Ch_(out,1), and a second output configuration forfrequency subband signals 1525-2 of a second output audio channel1535-2, Ch_(out,2). In the embodiment of FIG. 15, the feedback delayloop processor 1520 may be configured so that the first and secondoutput configurations may comprise connections 1527 to different filtertap positions or phase modification units. Specifically, in the FIG. 15embodiment, the first and second output configurations can be connectedto the same delay line 1526.

Essentially, by using the same delay line 1526 for providing differentlydelayed frequency subband signals for the output audio channelsgenerated from one input frequency subband signal, a number of thenecessitated delay lines within the feedback delay loop processor 1520can efficiently be reduced as compared to the case when two differentdelay lines are used for providing the differently delayed frequencysubband signals generated from the one input frequency subband signal.

According to further embodiments, the feedback delay loop processor mayalso comprise a first input configuration for frequency subband signalsof a first input audio channel and a second input configuration forfrequency subband signals of a second input audio channel. In suchembodiments, the feedback delay loop processor may be configured so thatthe first and second input configurations may comprise connections todifferent filter tap positions or phase modification units. Thereby, thefirst and second input configurations can be connected to a same delayline.

In embodiments, the number of input (Ch_(in,1), Ch_(in,2), . . . ) andoutput audio channels (Ch_(out,1), Ch_(out,2), . . . ) can be the sameor be different.

Essentially, the reverberator 1500 of the FIG. 15 embodiment provides areverberation algorithm being operative in the frequency domain, whichis based on a subband-wise processing of two or more channels of theaudio signal. As depicted in FIG. 15, the channel specific outputvectors may, for example, be configured to be orthogonal to each other.Here, a channel specific output vector may be defined by the specificdelay line outputs (pickup points or filter tap positions), which areused for the synthesis by a respective output processor. Referring tothe FIG. 15 embodiment, the channel specific output vectors areorthogonal with respect to each other, because for the first and thesecond channel, different pickup points or filter tap positions 1522,1524 can be used, respectively.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, some one or moreof the most important method steps may be executed by such an apparatus.

The inventive processed audio signal can be stored on a digital storagemedium or can be transmitted on a transmission medium such as a wirelesstransmission medium or a wired transmission medium such as the Internet.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, an EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. The data carrier, the digital storagemedium or the recorded medium are typically tangible and/ornon-transitionary.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods are performed by any hardware apparatus.

The above described embodiments are merely illustrative for theprinciples of the present invention. It is understood that modificationsand variations of the arrangements and the details described herein willbe apparent to others skilled in the art. It is the intent, therefore,to be limited only by the scope of the impending patent claims and notby the specific details presented by way of description and explanationof the embodiments herein.

The present invention essentially provides a novel, computationallyefficient structure for a reverberator which can be operative in afrequency transform domain. The benefits include an efficientimplementation compared to existing frequency domain solutions andarbitrary control for reverberation times in frequency bands.

Embodiments of the present invention can be based on an algorithm, whichoperates in the frequency transform domain and has an individual processin each subband. Moreover, the impulse response of this algorithm may beinfinitely repeating while exponentially attenuating in each frequencyband.

In the following, the main benefits of embodiments of the presentinvention are described. The presented solution produces reverberationthat is perceptually very close to the infinite frequency band-wiseexponentially decaying white noise, which is considered to be a goodreference for real diffuse reverberation. Moreover, the computationalcomplexity of the provided system is very small, which is also the casefor long reverberation times. In particular, an example implementationfor processing all subbands needed only 2.2 real multiplications and 10to 40 real additions (depending on the parameter T₆₀) per time domainsample.

The presented solution also allows completely free adjustment of theparameter T₆₀ individually in all frequency bands. This is importantespecially for room modeling and virtual acoustics since the parameterT₆₀ in frequency bands is an important property for human listeners inperceiving spaces, and in fact is a common measure in room acousticmeasurements and simulation. Finally, the present solution works in thefrequency domain.

There are numerous modern audio processing technologies that have ademand for a good quality frequency domain reverberation algorithm.

In the following, some of the use cases of embodiments of the presentinvention are described. A use case relates to adding room effect inapplications that function in a short-time frequency transform domain.An example of such applications are binaural decoding of MPEG Surroundas described in “Multi-Channel Goes Mobile: MPEG Surround BinauralRendering”, Breebaart, Herre, Jun, Kjörling, Koppens, Plogsties,Villemoes, 29 th AES conference, September 2006 and MPEG Surroundstandard ISO/IEC FDIS 23003-1, and SAOC as described in “Spatial AudioObject Coding (SAOC)—The Upcoming MPEG Standard on Parametric ObjectBased Audio Coding”, Breebaart, Engdegård, Falch, Hellmuth, Hilpert,Hoelzer, Koppens, Oomen, Resch, Schujiers, Trentiev. These decodersbenefit in having room effect in the hybrid QMF domain. The necessity ofreverberators is motivated by the necessity to create natural listeningexperience to the listener using headphones. Another use case relates toupmixing. Similarly to binaural decoding, upmixing applications oftenwork also in the frequency domain and may also use reverberators.Another use case relates to auralization in room acoustic design. Roomacoustic software needs a reverberator with free control of T₆₀ toauralize the space (such as a concert hall) in the design phase. Anotheruse case relates to game audio and VR. Successful creation of immersiveexperiences in virtual reality may depend on the ability to reproduceany given set of parameters T₆₀ correctly. Finally, another use caserelates to audio effects. The proposed technique may overcome somelimitations of the time domain reverberators. With help of a frequencytransform and an inverse frequency transform operation, the proposedtechnique may be applied as an effect in sound design.

While this invention has been described in terms of several advantageousembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andcompositions of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

The invention claimed is:
 1. Reverberator for reverberating an audiosignal, comprising: a feedback delay loop processor for delaying atleast two different frequency subband signals comprising a firstfrequency subband signal being a first time-frequency tile and adifferent second frequency subband signal being a second differenttime-frequency tile, the at least two different frequency subbandsignals representing the audio signal by different loop delay amounts toacquire at least two reverberated frequency subband signals comprising afirst reverberated frequency subband signal corresponding to the firsttime-frequency tile and a different second reverberated frequencysubband signal corresponding to the second time-frequency tile, whereinthe feedback delay loop processor comprises: for the first frequencysubband signal of the at least two frequency subband signals, a firstdelay line comprising a first plurality of delay line taps providingfirst signals delayed by first tap delays, a first feedback loopconnected to the first delay line and a first combiner for combining thefirst signals output by the first plurality of delay line taps to obtainthe first reverberated frequency subband signal corresponding to thefirst time-frequency tile, and for the second different frequencysubband signal of the at least two frequency subband signals, a seconddelay line comprising a second plurality of delay line taps providingsecond signals delayed by second tap delays, a second feedback loopconnected to the second delay line and a second combiner for combiningthe second signals output by the second plurality of delay line taps toobtain the second reverberated frequency subband signal corresponding tothe second time- frequency tile, wherein the first feedback looptogether with the first delay line represent a first loop delay amountand wherein the second feedback loop together with the second delay linerepresent a second loop delay amount, the second loop delay amount beingdifferent from the first loop delay amount, wherein the first delay lineand the first combiner are configured as a first sparse filter, andwherein the second delay line and the second combiner are configured asa second sparse filter, wherein the first sparse filter comprises afirst filter tap density, wherein a first filter impulse response of thefirst sparse filter approximates a first predetermined energy envelope,and wherein the second sparse filter comprises a second filter tapdensity, wherein a second filter impulse response of the second sparsefilter approximates a second predetermined energy envelope, wherein thefirst sparse filter or the second sparse filter comprises a plurality ofphase modification units, wherein each phase modification unit of theplurality of phase modification units is directly connected to anindividual delay line tap of the plurality of delay line taps, andwherein each phase modification unit is configured to apply a phaseoperation to a corresponding signal output by the individual delay linetap, the phase operation comprising a phase amount equal to k×π/2,wherein k is an integer number.
 2. The reverberator according to claim1, further comprising an output processor for processing first andsecond the reverberated frequency subband signals to aqcuire areverberated audio signal.
 3. The reverberator according to claim 1,wherein the output processor performs mixing the at least two frequencysubband signals and the corresponding first and second reverberatedfrequency subband signals to acquire mixed signals and to combine themixed signals, or combining the first and second reverberated frequencysubband signals to acquire the reverberated audio signal comprising acombined bandwidth.
 4. The reverberator according to claim 1, whereinthe feedback delay loop processor comprises, for each frequency subbandsignal, a filter comprising a filter impulse response, wherein thefilter impulse response comprises a first block of filter impulseresponse samples and a second block of filter impulse response samples,the second block being similar to the first block with regard to impulseresponse sample spacing, wherein the first impulse response sample ofthe second block is delayed from the first impulse response sample ofthe first block by the loop delay for the frequency subband signal, andwherein the first blocks and the second blocks of the filter impulseresponses of the filters for the frequency subband signals are delayedby the different loop delays.
 5. The reverberator according to claim 1,wherein the feedback delay loop processor comprises a feedback loop foreach frequency subband signal of the at least two frequency subbandsignals, wherein the feedback loop for a frequency subband signalcomprises a delay element and an attenuator, wherein the delay elementsare different with respect to their loop delays for the at least twodifferent frequency subband signals.
 6. The reverberator according toclaim 1, wherein the feedback delay loop processor is configured so thatthe second loop delay for a second frequency subband signal of the atleast two frequency subband signals representing a lower frequency bandis larger than the first loop delay for a first frequency subband signalof the at least two frequency subband signals representing a higherfrequency band.
 7. The reverberator according to claim 1, wherein thefirst delay line and the first combiner are configured as a first sparsefilter, and wherein the second delay line and the second combiner areconfigured as a second sparse filter, wherein the first sparse filtercomprises a first filter tap density, wherein a first filter impulseresponse of the first sparse filter approximates a first predeterminedenergy envelope, and wherein the second sparse filter comprises a secondfilter tap density, wherein a second filter impulse response of thesecond sparse filter approximates a second predetermined energyenvelope.
 8. The reverberator according to claim 1, wherein the audiosignal comprises a plurality of different input or output audiochannels, wherein each input or output audio channel comprises at leasttwo different frequency subband signals, wherein the feedback delay loopprocessor comprises a delay line filter, a delay line of the delay linefilter comprising filter tap positions or phase modification unitsconnected to at least some of the filter tap positions, the feedbackdelay loop processor further comprising a first input or outputconfiguration for frequency subband signals of a first input or outputaudio channel and a second input or output configuration for frequencysubband signals of a second input or output audio channel, and whereinthe feedback delay loop processor is configured so that the first andsecond input or output configurations comprise connections to differentfilter tap positions or phase modification units, and wherein the firstand second input or output configurations are connected to the samedelay line.
 9. The reverberator according to claim 1, wherein thefeedback delay loop processor performs attenuating each frequencysubband signal of the at least two frequency subband signals by anattenuation factor, wherein the attenuation factor depends on apredetermined reverberation time and the loop delay for the frequencysubband signal.
 10. The reverberator according to claim 1, wherein thefirst delay line and the second delay line are complex-valued devices,and wherein each phase modification unit of the plurality of phasemodification units comprises a first phase modification unit input for areal part of a respective delay line tap output signal and a secondphase modification unit input for an imaginary part of the respectivedelay line tap output signal, wherein each phase modification unit ofthe plurality of phase modification units a first phase modificationunit output for the real part of a phase modified output signal and asecond phase modification unit output for the imaginary part of thephase modified output signal, wherein a phase modification unit isconfigured so that the first phase modification unit input is directlyconnected to the first phase modification unit output and the secondphase modification unit input is directly connected to the second phasemodification unit output, the first phase modification unit input isconnected to a sign inverter and the sign inverter is connected to thesecond phase modification unit output and the second phase modificationunit input is directly connected to the first phase modification unitoutput, the first phase modification unit input is connected to a firstsign inverter and the first sign inverter is connected to the firstphase modification unit output and the second phase modification unitinput is connected to a second sign inverter and the second signinverter is connected to the second phase modification unit output, orthe first phase modification unit input is directly connected to thesecond phase modification unit output and the second phase modificationunit input is connected to a sign inverter and the sign inverter isconnected to the first phase modification unit output.
 11. Thereverberator of claim 1, further comprising: a spectral converter forconverting the audio signal into a spectral representation comprisingthe first frequency subband signal and the second frequency subbandsignal.
 12. The reverberator according to claim 1, further comprising: atime-to-frequency converter comprising an analysis filterbank, aquadrature mirror analysis filterbank, a modified discrete cosinetransform, or a short time Fourier transform for generating the firstand the second frequency subband signals from the audio signal, or afrequency-to-time converter comprising a synthesis filterbank, aquadrature mirror synthesis filterbank an inverse modified discretecosine transform, or an inverse short time Fourier transform forcombining the first and the second frequency subband signals.
 13. Amethod for reverberating an audio signal, comprising: delaying at leasttwo different frequency subband signals comprising a first frequencysubband signal being a first time-frequency tile and a different secondfrequency subband signal being a second different time-frequency tile,the at least two different frequency subband signals representing theaudio signal. by different loop delay amounts by using a feedback delayloop processor to acquire at least two reverberated frequency subbandsignals comprising a first reverberated frequency subband signalcorresponding to the first time-frequency tile and a different secondreverberated frequency subband signal corresponding to the secondtime-frequency tile, wherein the feedback delay loop processorcomprises: for the first frequency subband signal of the at least twofrequency subband signals a first delay line comprising a plurality ofdelay line taps providing first signals delayed by first tap delays, afirst feedback loop connected to the first delay line and a firstcombiner for combining the first signals output by the plurality ofdelay line taps to obtain the first reverberated frequency subbandsignal corresponding to the first time-frequency tile, and for thesecond different frequency subband signal of the at least two frequencysubband signals, a second delay line comprising a second plurality ofdelay line taps providing signals delayed by second tap delays, a secondfeedback loop connected to the second delay line and a second combinerfor combining the second signals output by the second plurality of delayline taps to obtain the second reverberated frequency subband signalcorresponding to the second time- frequency tile, wherein the firstfeedback loop together with the first delay line represent a first loopdelay amount and wherein the second feedback loop together with thesecond delay line represent a second loop delay amount, the second loopdelay amount being different from the first loop delay amount, whereinthe first delay line and the first combiner are configured as a firstsparse filter, and wherein the second delay line and the second combinerare configured as a second sparse filter, wherein the first sparsefilter comprises a first filter tap density, wherein a first filterimpulse response of the first sparse filter approximates a firstpredetermined energy envelope, and wherein the second sparse filtercomprises a second filter tap density, wherein a second filter impulseresponse of the second sparse filter approximates a second predeterminedenergy envelope, wherein the first sparse filter or the second sparsefilter comprises a plurality of phase modification units, wherein eachphase modification unit of the plurality of phase modification units isdirectly connected to an individual delay line tap of the plurality ofdelay line taps, and wherein each phase modification unit is configuredto apply a phase operation to a corresponding signal output by theindividual delay line tap, the phase operation comprising a phase amountequal to k×π/2, wherein k is an integer number.
 14. The method of claim13, further comprising: converting the audio signal into a spectralrepresentation comprising the first frequency subband signal and thesecond frequency subband signal.
 15. A non-transitory storage mediumhaving stored thereon a computer program comprising a program code forperforming, when the computer program is executed on a computer, amethod for reverberating an audio signal, the method comprising:delaying at least two different frequency subband signals comprising afirst frequency subband signal being a first time-frequency tile and adifferent second frequency subband signal being a second differenttime-frequency tile, the at least two different frequency subbandsignalsrepresenting the audio signal by different loop delay amounts byusing a feedback delay loop processor to acquire at least tworeverberated frequency subband signals comprising a first reverberatedfrequency subband signal corresponding to the first time-frequency tileand a different second reverberated frequency subband signalcorresponding to the second time-frequency tile, wherein the feedbackdelay loop processor comprises: for the first frequency subband signalof the at least two frequency subband signals a first delay linecomprising a first plurality of delay line taps providing first signalsdelayed by first tap delays, a first feedback loop connected to thedelay line and a first combiner for combining the first signals outputby the first plurality of delay line taps to obtain the firstreverberated frequency subband signal corresponding to the firsttime-frequency tile, and for the second different frequency subbandsignal of the at least two frequency subband signals, a second delayline comprising a second plurality of delay line taps providing secondsignals delayed by second tap delays, a second feedback loop connectedto the second delay line and a second combiner for combining the secondsignals output by the second plurality of delay line taps to obtain thesecond reverberated frequency subband signal corresponding to the secondtime- frequency tile, wherein the first feedback loop together with thefirst delay line represent a first loop delay amount and wherein thesecond feedback loop together with the second delay line represent asecond loop delay amount, the second loop delay amount being differentfrom the first loop delay amount, wherein the first delay line and thefirst combiner are configured as a first sparse filter, and wherein thesecond delay line and the second combiner are configured as a secondsparse filter, wherein the first sparse filter comprises a first filtertap density, wherein a first filter impulse response of the first sparsefilter approximates a first predetermined energy envelope, and whereinthe second sparse filter comprises a second filter tap density, whereina second filter impulse response of the second sparse filterapproximates a second predetermined energy envelope, wherein the firstsparse filter or the second sparse filter comprises a plurality of phasemodification units, wherein each phase modification unit of theplurality of phase modification units is directly connected to anindividual delay line tap of the plurality of delay line taps, andwherein each phase modification unit is configured to apply a phaseoperation to a corresponding signal output by the individual delay linetap, the phase operation comprising a phase amount equal to k×π/2,wherein k is an integer number.
 16. The non-transitory storage medium ofclaim 15, wherein the method further comprises converting the audiosignal into a spectral representation comprising the first frequencysubband signal and the second frequency subband signal.